This invention relates to a method and apparatus for the separation of solutes dissolved in gaseous solvents. In particular this invention relates to the separation and recovery of one or more solutes dissolved in high pressure gases and the recovery of the purified gas at high pressure. For the purposes of this invention, a gaseous solution is considered to be a solution where the solvent or the solution has significant compressibility.
At low pressure gases are poor solvents, and the potential solubility of a potential solute is limited to its volatility. For example the moisture content of air is limited by the vapor pressure of water at the dew point, which is the temperature at which the vapor pressure of water equals the partial pressure of water in the gas. At high pressure gases can have significant solvent action even for non-volatile substances, for example high pressure steam can dissolve silica at temperatures where the vapor pressure of silica is negligible. See FIG. 4.
The solvent action of a particular gas has a strong dependance on gas density as influenced by pressure or temperature, where a particular gas at high density can dissolve more solute than the same gas at a lower density. This strong effect of density or pressure on solute solubility is well known and is used in the field of supercritical fluid extraction, which is well described by Krukonis and McHugh, in their text "Supercritical Fluid Extraction" Butterworth Publishers 1986, and by Zosel U.S. Pat. No. 3,969,196.
In a typical SCF process a solute is extracted from a substrate using a high density, high pressure gaseous solvent, the solvent is then reduced in pressure (and hence density) to where the concentration of the solute now exceeds the new solubility and the solute precipitates out and can be mechanically recovered from the gaseous solvent which now can be compressed and reused. This typical process is well described by Krukonis and McHugh and earlier by Zosel.
There are drawbacks to this type of density reduction by pressure reduction, and also with the corresponding density reduction by temperature increase process, namely:
1. Significant solute remains dissolved unless the pressure is taken to very low levels, necessitating large compression costs for solvent reuse. PA1 2. Formation of 2 phases in pressure reduction or temperature adjustment equipment can lead to erosion, or reduction of heat transfer coefficients.
There are also cases where a reduction in the pressure of the high pressure gas is not feasible. For example in the production of high pressure steam. Silica is an ubiquitous contaminant in ground and surface water. In a boiler the high pressure steam dissolves some silica and carries it through the system until the turbine where the steam is expanded to low pressure. The silica becomes less soluble in the low pressure steam and precipitates as a solid inside the turbine causing degradation of performance. The solubility of silica and other minerals in steam necessitates extreme purity in boiler feed water and precludes the open cycle use of geothermal steam or steam from supercritical water oxidation as described by Modell U.S. Pat. No. 4,338,199 or Dickinson U.S. Pat. No. 4,292,953. Work is only extracted from the steam by expansion to a lower pressure where silica precipitation is inevitable.
Petroleum can be recovered from underground formations by using high pressure carbon dioxide as a solvent. The expansion, phase separation and recompression of the gas can be expensive and may not recover many of the lighter hydrocarbons, unless expansion to a very low pressure is utilized, which increases compression costs.
SCF processing is used for many separation applications, mostly involving high value materials like specialty food stuffs, such as decaffeinated coffee, where incidentally the solute caffeine is typically not recovered, but is instead irreversibly adsorbed onto activated carbon.
A major cost in SCF processing is the energy needed to compress the fluid so as to restore its solvent properties after expansion to remove solute. A sorbent such as activated carbon can be used to remove solute while avoiding a pressure reduction. Where the desired product is the solute, for example in oil seed extraction or for spice extraction then a pressure change and the resulting energy cost is unavoidable.
Work is a force times a distance. The work of compression of a fluid depends to a great extent on the compressibility of the fluid and the volume change that occurs during compression. Liquids have low compressibilities and require little work to raise their pressure. Gases on the other hand because of their compressibility require much more work to compress. Compression of a dense gas through a modest density change can require less work than compression of the low pressure gas through a larger volume change, even though the pressure change, or the maximum pressure may be lower.
High pressure gases can dissolve solutes in processes where this effect is undesirable. For example in the compression of gases the lubricants from the compressor can contaminate the gas. In the case of carbon dioxide to be used for semiconductor cleaning, separation of lubricants or other low volatility contaminants entails distillation. Distillation of carbon dioxide must occur in the subcritical two phase region. To obtain supercritical carbon dioxide the fluid must then be compressed and heated. In the case where the solute is a solid, distillation results in the formation of solid in the distillation column which can cause plugging and inefficient operation. Similarly fluids used in cryogenic refrigeration cycles must be cleaned of solutes prior to expansion so as to avoid solid formation and concentration of oil in the evaporator and inadequate lubrication of the compressor.
Supercritical fluid chromatography in particular when using pressure programming can be used to separate a great many compounds, especially hydrocarbons on an analytical scale. Extremely pure fluids are needed, and compressor lubricants cannot be easily removed.